1. Field of the Invention
The present invention relates to Doppler speed sensors.
2. Description of the Prior Art
Doppler radar speed sensors for ground vehicles are typically mounted on the underside of the vehicle such that a probe signal is radiated towards the ground surface at a predetermined angle of incidence. A portion of the probe signal is reflected retrodirectively from the ground surface back in the direction from which the probe signal came. Such retrodirectively reflected portion of the probe signal is referred to in the art as "backscatter." The remainder of the probe signal is either absorbed by the ground surface or reflected, other than retrodirectively, at various angles, and in particular, at an angle of reflection equal to the angle of incidence but on the opposite side of the normal to the ground surface. The amount of backscatter from a given ground surface, is affected by the parameters and radiation pattern of the probe signal, and by the characteristics of the ground surface. For example, the wavelength, power and polarization of the radar probe, the area of ground surface illuminated by the probe signal, the angle of incidence of the probe signal, and the conductivity, permittivity and roughness of the ground surface all affect backscatter from the ground surface.
The frequency of the backscatter is Doppler shifted from the frequency of the probe signal (f.sub.o) in accordance with the speed of relative motion between the vehicle and the ground surface. The backscatter is received by the radar, and the Doppler frequency shift thereof is determined to derive the relative speed.
It is desirable that the output signal of a vehicular Doppler speed sensor manifest only the forward velocity of the vehicle, that is, the value of the component of velocity in a forward direction parallel to the longitudinal axis of the vehicle and parallel to the ground surface (hereinafter termed "parallel velocity"). However, the Doppler speed sensor output signal manifests the magnitude of the relative velocity, and, the Doppler shift of the backscatter typically includes components attributable to relative motion between the sensor and the ground surface in directions perpendicular (normal) to the ground surface, in addition to components indicative of the parallel velocity. Such perpendicular motion may be caused by, for example, bounce of the vehicle. Thus, Doppler shift due to components of velocity in a direction perpendicular to the ground surface (hereinafter termed "perpendicular velocity") introduce an error into the system. A similar error is caused in prior plural antenna systems, described below, by non-parallel alignment ("tilt") of the vehicle underside with respect to the ground surface.
The sensitivity of the radar to perpendicular velocity, and hence the error therefrom, increases as the angle of probe incidence approaches the normal to the ground surface. For example, the prior art systems typically operate with angle of incidence of 40.degree. with respect to the ground surface. If an angle of incidence of 80.degree. with respect to the ground surface were utilized, a perpendicular velocity of 1/20 (5%) of the parallel velocity will result in an error of 30 percent. Additional errors due to vehicle tilt in plural antenna systems also increase as the angle of incidence approaches normal.
A dilemma arises in vehicular speed sensors due to the increase in errors due to perpendicular velocity and vehicle tilt as the angle of incidence approaches normal, in that the amount of backscatter generated by a probe signal decreases as the angle of incidence moves away from normal to the ground surface. In automotive speed sensors, the characteristics of many road surfaces, particularly when wet, tend to generate very little backscatter, requiring that an angle of incidence relatively close to normal be used to generate adequate amounts of backscatter. Thus, the requirement of a low angle of incidence to avoid errors due to vehicle vertical velocity is in conflict with the requirement of a high angle of incidence for production of backscatter.
In prior art systems, attempts have been made to increase backscatter without increasing the angle of incidence towards normal by, for example, placing the Doppler sensor directly following a wheel, the wheel serving presumably to clear the road surface of water and mud. Such systems are disadvantageous in that the splash of water and mud not only may damage the equipment but also affects the transmission property of the signals.
Other prior art systems, utilizing a plurality of transmitting and receiving antennas have been proposed. For example, a so-called "Janus" system, is described in an article entitled "Vehicular Radar Speedometer" by Hyltin, Fuchser, Tyson and Regueiro presented before the Society of Automotive Engineers at the International Automotive Engineering Congress and Exposition, Jan. 8-12, 1973. A similar system is described in U.S. Pat. No. 3,833,906 issued Sept. 3, 1974, to C. F. Augustine. The Janus-type systems comprise two "back-to-back" antennas, respectively facing forward and rearward with non-overlapping radiation patterns, each antenna serving to provide transmitting and receiving functions. The respective backscatter signal portions received by the antennas are added and subtracted to derive sum and difference signals, from which the Doppler speed is determined. The Janus system tends to decrease errors due to perpendicular velocity and vehicle tilt.
However, the back-to-back Janus configuration of the antennas creates an upper limit on how closely the angles of incidence can approach normal. For example, the angle of incidence in such Janus systems is limited in that the radiation patterns of the respective antennas have finite widths and any overlap of the respective radiation patterns typically causes interference and errors. In addition, the Janus configuration tends to cause non-specular ground radiation from one antenna to be received by the other, causing further errors. Moreover, the processing circuitry of such prior art systems, typically either operate at the relatively low ampltiude levels of backscatter signals, or include relatively complex amplification means.